A Time Dependent, Modal Reduced-Order Model node (
) contains settings for a reduced-order model that uses a time-domain modal method. See
Modal Reduced-Order Models.
Click Export Reduced Model (
) to save the reduced-order model to file as COMSOL Reduced Model file (MPHROM-file). Note that export is only possible if all defined outputs are linear quantities.
The Settings window for a
Time Dependent, Modal, Reduced-Order Model node contains the following sections:
Here you specify whether the reduced-order model should have stateless or stateful interface. Choose Stateless (the default) or
Stateful from the
Interface list. In the former case, the reduced-order model acts as a black box that uses an internal solver and does not expose its state. In the latter case, the reduced-order model exposes a set of reduced-order equations and state DOFs to the solver used for the calling model, and you can choose whether to solve for the reduced-order model in the same way as for a physics interface.
If you choose Stateless, also specify the
Time value to be used when evaluating outputs. It is by default set to
t, which means that the reduced-order model will be evaluated for the same time as in the calling model in which it is used.
If you choose Stateful, from the
Equation form list, choose
Study controlled (the default) or
Time domain. When the setting is
Study controlled, the study controls the equation form for the reduced-order model. It then appears as
Automatic (Time dependent), for example, in the
Physics and Variables Selection section in the study step’s Settings window.
The list in this section contains the reduced-order model input quantities that were set as active in the corresponding Model Reduction node’s
Model Control Inputs section when the reduced-order model was created. For each model control input in the
Variable column, specify a corresponding
Expression to be used when evaluating outputs from the reduced-order model, or when using the stateful interface, when solving reduced-order equations. The default expression is the name of the corresponding reduced-order model input variable. The effect of this is that the reduced-order model will be evaluated for the same value of the input as is seen by the calling model in which it is used. This is usually the desired behavior.
The constraints defined by the settings in the Constraint Modes table are applied in a symmetric way directly on the constraint mode states. This has a few important implications:
When the Use output dependent variables check box is selected, the reduced-order model also declares the dependent variable names entered in the
Dependent variable column. When the reduced-order model is part of another model, these dependent variables can be assigned the value of the corresponding output variable each time a solution is stored. If the reduced-order model has a stateless interface, this behavior is controlled by the
Store output dependent variables setting in the
Physics and Variables Selection section of each study step where the reduced-order model is used. If the reduced-order model has a stateful interface, this behavior is controlled by whether the reduced-order model is solved for in a given study step. Apart from the setting to solve for a reduced-order model with a stateful interface, these settings are only available for reduced-order models that have outputs.
Use the Load factor to scale the constant part of the right-hand side of the unreduced model. This in practice scales all loads and source terms with the specified factor. Inhomogeneous constraints are not affected.
If the reduced-order model has a stateless interface, specify a Relative tolerance for the BDF time-stepping method used in the internal solver. This setting is only available when
Stateless is selected from the
Interface list in the
Usage section above.
Under Matrices, all matrices that the reduced model solution includes are listed. The listed matrices can be accessed using the COMSOL API.
Under Vectors, all vectors that the reduced model solution includes are listed. The listed vectors can be accessed using the COMSOL API.
If the second derivatives are zero, and Brdot (the time-derivative input matrix) is zero, let
Kud denote the stiffness matrix
K times
Ud, which is a particular solution satisfying the constraints. Then, the state-space equations can be written in this first-order form:
where B0r is the initial value input matrix.
where Y0 is the output bias vector.
The eliminated MAxe and Cxe vectors are available for output (see the table below).
Here, Ir in the second-order definition represents the identity matrix.
To access these matrices and vectors, right-click Results>Derived Values, choose
System Matrix, and in the settings for the
System Matrix node, choose
Time Dependent, Modal Reduced-Order Model from the
Solution list. The
Matrix list in the
Output section then includes all matrices and vectors of interest.